Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational October 19, 2018
Expires: April 22, 2019
Extensions to Automatic Certificate Management Environment for end user
S/MIME certificates
draft-ietf-acme-email-smime-04
Abstract
This document specifies identifiers and challenges required to enable
the Automated Certificate Management Environment (ACME) to issue
certificates for use by email users that want to use S/MIME.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Use of ACME for issuing end user S/MIME certificates . . . . 2
3.1. ACME challenge email . . . . . . . . . . . . . . . . . . 3
3.2. ACME response email . . . . . . . . . . . . . . . . . . . 4
4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
[I-D.ietf-acme-acme] is a mechanism for automating certificate
management on the Internet. It enables administrative entities to
prove effective control over resources like domain names, and
automates the process of generating and issuing certificates.
This document describes an extension to ACME for use by S/MIME.
Section 3 defines extensions for issuing end user S/MIME [RFC5750]
certificates.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Use of ACME for issuing end user S/MIME certificates
[I-D.ietf-acme-acme] defines "dns" Identifier Type that is used to
verify that a particular entity has control over a domain or specific
service associated with the domain. In order to be able to issue
end-user S/MIME certificates, ACME needs a new Identifier Type that
proves ownership of an email address.
This document defines a new Identifier Type "email" which corresponds
to an (all ASCII) email address [RFC5321] or Internationalized Email
addresses [RFC6531]. This can be used with S/MIME or other similar
service that requires posession of a certificate tied to an email
address.
Any identifier of type "email" in a newOrder request MUST NOT have a
wildcard ("*") character in its value.
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A new challenge type "email-reply-00" is used with "email" Identifier
Type, which provides proof that an ACME client has control over an
email address:
1. ACME server generates a "challenge" email message with the
subject "ACME: ", where is the
base64url encoded first part of the token, which contains at
least 64 bit of entropy. The challenge email message structure
is described in more details in Section 3.1. The second part of
the token (token-part2, which also contains at least 64 bit of
entropy) is returned over HTTPS [RFC2818] to the ACME client.
2. ACME client concatenates "token-part1" and "token-part2" to
create "token", calculates key-authz (as per Section 8.1 of
[I-D.ietf-acme-acme]), then includes the base64url encoded
SHA-256 digest [FIPS180-4] of the key authorization in the body
of a response email message containing a single text/plain MIME
body part [RFC2045]. The response email message structure is
described in more details in Section 3.2
For an identifier of type "email", CSR MUST contain the request email
address in an extensionRequest attribute [RFC2985] requesting a
subjectAltName extension.
3.1. ACME challenge email
A "challenge" email message MUST have the following structure:
1. The message Subject header field has the following syntax: "ACME:
", where the prefix "ACME:" is followed by exactly
one SP character. is the base64url encoded first
part of the ACME token that MUST be at least 64 octet long after
decoding. [[Alexey: is the following going to be problematic due
to the total subject lenght and recommended 78 octet line length
limit?]] For ease of implementation, no RFC 2231 encoding of
subject is allowed in the "challenge" email message.
2. The message MUST include the "Auto-Submitted: auto-generated"
header field [RFC3834]. It MAY include optional parameters as
allowed by syntax of Auto-Submitted header field.
3. The message MUST have a single text/plain MIME body part
[RFC2045], that contains human readable explanation of the
purpose of the message.
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Example ACME "challenge" email
Auto-Submitted: auto-generated
Date: Sat, 1 Sep 2018 10:08:55 +0100
Message-ID:
From: acme-generator@example.org
To: alexey@example.com
Subject: ACME:
Content-Type: text/plain
MIME-Version: 1.0
This is an automatically generated ACME challenge for email address
"alexey@example.com". If you haven't requested an S/MIME
certificate generation for this email address, be very afraid.
If you did request it, your email client might be able to process
this request automatically, or you might have to paste the first
token part into an external program.
Figure 1
3.2. ACME response email
A "response" email message MUST have the following structure:
1. The message Subject header field has the following syntax: "Re:
ACME: ", where the string "ACME:" is followed by
exactly one SP character. is the base64url encoded
first part of the ACME token. (Note that this would be a subject
generated by any email client replying to the "challenge" email
message.) [[Alexey: is the following going to be problematic due
to the total subject lenght and recommended 78 octet line length
limit?]] For ease of implementation, no RFC 2231 encoding of
subject is allowed in the "response" email message.
2. The From: header field contains the email address of the user
that is requesting S/MIME certificate issuance.
3. The To: header field of the response contains the value from the
From: header field of the challenge email.
4. The Cc: header field must be absent in the "response" email
message.
5. The message MUST have a single text/plain MIME body part
[RFC2045], containing base64url encoded SHA-256 digest
[FIPS180-4] of the key authorization, calculated based on token-
part1 (received over email) and token-part2 (received over
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HTTPS). Note that due to historic line length limitations in
email, line endings (CRLFs) can be freely inserted in the middle
of the encoded digest, so they need to be ignored when processing
it.
6. There is no need to use any Content-Transfer-Encoding other than
7bit, however use of Quoted-Printable or base64 is not prohibited
in a "response" email message.
Example ACME "response" email
Date: Sat, 1 Sep 2018 11:12:00 +0100
Message-ID: <111-22222-3333333@example.com>
From: alexey@example.com
To: acme-generator@example.org
Subject: Re: ACME:
Content-Type: text/plain
MIME-Version: 1.0
LoqXcYV8q5ONbJQxbmR7SCTNo3tiAXDfowy
jxAjEuX0.9jg46WB3rR_AHD-EBXdN7cBkH1WOu0tA3M9
fm21mqTI
Figure 2
4. Open Issues
[[This section should be empty before publication]]
1. Do we need to handle text/html or multipart/alternative in email
challenge? Simplicity suggests "no". However, for automated
processing it might be better to use at least multipart/mixed
with a special MIME type.
2. How to verify authenticity of "challenge" email messages? The
document can recommend either S/MIME signing (in which case the
"challenge" email message should be allowed to be S/MIME signed
multipart/signed or application/pkcs7-mime containing SignedData)
and/or presence of DKIM signature covering at least From/To/
Subject, with working SPF and DMARC.
3. Similarly to the above: how to verify authenticity of "response"
email messages? We can't require use of S/MIME, as this protocol
is used for S/MIME user enrollment. However DKIM/SPF/DMARC can
be recommended.
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4. Define a new parameter to "Auto-Submitted: auto-generated", so
that it is easier to figure out that a particilar message is an
ACME challenge message?
5. IANA Considerations
IANA is requested to register a new Identifier Type "email" which
corresponds to an (all ASCII) email address [RFC5321] or
Internationalized Email addresses [RFC6531].
And finally, IANA is requested to register the following ACME
challenge types that are used with Identifier Type "email": "email-
reply". The reference for it is this document.
6. Security Considerations
TBD.
7. Normative References
[FIPS180-4]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4, August 2015,
.
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-14 (work in progress), August 2018.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
.
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[RFC2985] Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
Classes and Attribute Types Version 2.0", RFC 2985,
DOI 10.17487/RFC2985, November 2000,
.
[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, DOI 10.17487/RFC3834, August
2004, .
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
.
[RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Certificate
Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,
.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, .
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Appendix A. Acknowledgements
Thank you to Andreas Schulze and Gerd v. Egidy for suggestions,
comments and corrections on this document.
Author's Address
Alexey Melnikov
Isode Ltd
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail: alexey.melnikov@isode.com
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